This invention relates to the measurement of the permeability fields of underground geologic fluid systems and more specifically it uses microseismicity induced by the production process for determining the petrophysical properties of a reservoir in general, its permeability in particular and analyzing its structure.
It is generally known that the earth""s crust contains underground fluid reservoirs. These reservoirs form an important natural resource for major components of our economic systems, e.g. oil, gas, water, etc. Recovery of these resources is critically dependent on knowing the xe2x80x9cplumbingxe2x80x9d of these reservoirs, i.e. the paths through which the fluid moves and by means of which it can be extracted. In a fluid reservoir the xe2x80x9cplumbing systemxe2x80x9d includes a network of interconnected cracks which can be described as xe2x80x9chydraulically linkedxe2x80x9d i.e. changes in fluid pressure can be transmitted through them. The character of the hydraulically linked crack network is known as the xe2x80x9creservoir permeability fieldxe2x80x9d. By character is meant the shape and distribution of the network and the ease with which fluid moves through it. Determining this character is the focus of much of the effort of fluid resource recovery and exploitation.
The reason for this interest is that the spatial geometry of the permeability field and the variation in flow through it, are major factors in identifying the best location for production wells and in the case of hydrocarbons, injector wells in addition to production wells. Injector wells are used to inject fluids that are denser than the hydrocarbons and thus act to xe2x80x9csweepxe2x80x9d the less dense fluid that remains after the initial production phase.
To date, determination of the permeability field of fluid reservoirs has been largely restricted to the use of xe2x80x9cguess and testxe2x80x9d methods using reservoir simulators. A xe2x80x9cguess and testxe2x80x9d method uses largely inferential and sparse information about the permeability field to make a best xe2x80x9cguessxe2x80x9d as to its full three dimensional character. The xe2x80x9cguessxe2x80x9d is then tested by using measured data on production and injection from the field in question in the model, to test whether the model reproduces the measured data. The efficacy of the xe2x80x9cguess and testxe2x80x9d methods is poor. It is generally accepted that one of the principal reasons that recovery of resource from hydrocarbon reservoirs averages only about 30-35% of the total resource in any given reservoir, is the low quality of permeability information. What is needed is a means of seeing or illuminating the permeability field so that it can be directly measured.
It has been recognized that production from fluid reservoirs can induce seismicity. Attempts have been made to use microseismicity induced by production to identify fracture systems and other possible causes of the earthquakes, e.g. pore collapse, fault reactivation, etc. Efforts at deciphering the information contained in the microseismicity has had limited success. It is believed that the principal reason for this failure has to do with the way earthquake seismologists get their information on the earthquake process (seismogenesis).
An earthquake is a seismic wave that results from the elastic failure of rock. It is the signal or xe2x80x9csoundxe2x80x9d of that failure. However, elastic failure can result from a variety of natural processes, e.g. folding, faulting under compression/extension, pore collapse, increased fluid pressure, etc. Unfortunately all seismic waves produced by natural causes, no matter what their seismogenesis, are very similar in their appearance or xe2x80x9cwave formxe2x80x9d. This condition makes it virtually impossible to derive the particular process (i.e. folding, faulting, etc.) that produces any given seismic wave by studying the wave itself. Thus the almost exclusive restriction of the analysis of microseismicity to its signal has produced only a limited amount of useful information particularly as far as reservoir characteristics are concerned.
Attempts to distinguish the sets of signals from one another are largely ad hoc. Thus identification of fractures associated with the permeability field is; 1) only inferential; 2) limited to the immediate vicinity of injection wells (i.e. within a few 100 meters). This information is too poor to be usefully extrapolated to the entire reservoir. No attempts to directly measure any other components of the reservoir permeability field using microseismicity have been made. In terms of structural data, i.e. folding and faulting, microseismicity has been used to successfully locate faults in some cases where apparently only simple faulting is occurring. However regions with more complex deformation result in earthquake data clouds which are either left un-interpreted or only poorly explained.
A recent development that permits the clouds of earthquake data to be much more rigorously interpreted is described by Seeber and Armbruster (1995 The San Andreas Fault system through the Transverse Ranges as illuminated by earthquakes, J. Geophysical Research, 100, 5, 8285-8310). They have developed analysis techniques for earthquake slip planes that allow them to be sorted into structural assemblages. These structural assemblages represent portions of the instantaneous and incremental deformation field. The techniques for analyzing the slip plane data are embodied in a known software application; Seeber and Armbruster""s QuakeView.
In addition to the foregoing, the following sets of observations on secondary hydrocarbon recovery, hydraulically conductive fractures and microseismicity, are of particular importance with regard to the background of the present invention.
1. Rate Correlation Statistics, Maximum Compressive Directions and Rapid Response
Heffer et al, (1997, Novel techniques show links between reservoir flow directionality, earth stress, fault structure and geomechanical changes in mature waterfloods, SPE Journal, V. 2, June, pp. 91-98) show that rate of production correlation""s between producer and injection wells is directly related to the orientation of the maximum ambient compressive stress direction. Positive correlation""s (i.e. production increases) are observed between injection and production wells where the line connecting the two wells lies within a sector of arc of from 60 to 90 degrees that is bisected by the local maximum compressive stress. Response times between injector and producer wells has xe2x80x9czeroxe2x80x9d (less than 1 month) time lag over very large distances ( greater than 4.5 kilometers). They note that D""arcyian type diffusive flow cannot explain this phenomena.
2. Hydraulically Conductive Fractures are Critically Stressed
Barton et al (1995, Fluid flow along potentially active faults in crystalline rock, Geology, V. 23, no. 8, p. 683-686) demonstrate that critically stressed faults and fractures are those with the highest hydraulic conductivity and that statistically these are conoidally distributed around the maximum stress direction (Barton et al, 1995; FIG. 3).
3. Seismicity Induced By Increased Fluid Pressure Shows Rapid Response Over Large Distances
In the earthquake control experiment run at Chevron""s Rangely, Colo. field and reported by Raleigh et al (1976, An experiment in earthquake control at Rangely, Colo.; Science, V. 191, p. 1230-1237), microseismicity induced by fluid injection and occurring at distances of up to 3 km from the injection well, were observed to stop within 1 day of lowering fluid pressure at the injection wells.
Accordingly, several objects and advantages of my invention are to provide a method of using microseismicity to determine the nature of the interconnected network of openings that define the permeability field of underground reservoirs. By nature is meant the shape of this network, the variation of it""s shape in space and time, the change in space and time of the rate of movement of the fluid through the network, etc. The invention does this by providing a means of distinguishing microseismicity associated with the permeability field from other types of microseismicity. It also permits determination of the relationship of the microseismicity to other elements of the reservoir geology, e.g. rock type, whether part of a fold or fault, etc. This information can be used as input to reservoir models and other multi-dimensional images for exploration, production and development thereby improving the potential of recovery of fluid resources from the earth""s crust.
Other objects are;
1. to provide a means of guiding the placement of subsequent wells for the purposes of infill and/or development and/or injection;
2. to provide a means of determining the orientation of the maximum compressive stress direction using fluid injection;
3. to provide a means of using the information derived from the microseismically produced signal to be used for multidimensional analysis of petrophysical properties including direct 4D measurement of components of the permeability field and structural environments;
4. to improve velocity field models;
5. to improve interpretation of geodetic surveys whose aim is identifying subsurface behavior of the crust associated with fluid motion;
6. to provide independent evidence for paths of fluid motion that may be identified by 4D seismic reflection analysis or other means;
7. to provide information on possible hazards to human infrastructure arising from deformation of the earth""s crust associated with fluid extraction and injection.